The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
A telescopic mechanism of a medium or small tonnage telescopic boom type crane is usually formed by a hydraulic cylinder and a cable row, and such a mechanism allows several boom sections to extend or retract synchronously. Usually, for a small tonnage telescopic boom crane, synchronous extension and retraction can be implemented by only using two strands of cables on two sides of a boom section of the telescopic boom, that is, only one cable sheave is needed on each side of the boom section, and the cable sheave is located inside the boom section. Moreover, because of specifications of the curvature diameter (in a cable application, the diameter of a sheave is about 15 to 20 times of the diameter of the cable), a cable with a large diameter requires a sheave with the large diameter corresponding to the cable, and therefore occupies larger internal space of the telescopic boom, causing an increase of structure dimensions of both outer boom section and inner boom section. The increase of the structure dimension of the section of the telescopic boom means increases of both product weights and production costs.
For a medium tonnage telescopic boom crane, a manner in which sheaves are disposed abreast is usually used. In this case, the sheave is provided with two or three cable slots, and cables are arranged abreast along a widthwise direction of a boom section. By using the telescopic structure with such design, load of the one single cable is evenly distributed among the multiple strands of cables to achieve the effect of reducing the size of the cables while the loading or breaking force remains the same, thereby reducing the radial dimension of the sheaves. However, because the cables are arranged horizontally side by side, it takes larger space in a widthwise direction of the section of the telescopic boom. In design of a crane boom, the size of a base telescopic boom is relatively fixed due to loading requirement, a platform width, and transportation dimension. However, when a few of boom sections is installed, especially when a fourth and a fifth boom sections are installed the width space becomes very limited, and may cause serious loss of the rigidity in the widthwise direction of the telescopic boom.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies